As I just described in bug 707567, the main problem with double-clicking is that it could easily be triggered accidentally.

Middle-clicking is more plausible, since it doesn't have the same problem, or at least not nearly as much. The only drawback I can think of is that someone might click the middle button accidentally while changing volume with the scrollwheel, or vice versa.

I would not, however, want application indicator menus to be able to use middle click, because then developers would go off and make some functions available only that way and screw people who don't have a middle button. So if we can do it just for the sound menu without making it a general capability, I think that would be fine.